The present invention is directed to electrical modulating circuits, and especially to sigma delta modulating circuits employed for analog-to-digital signal conversion to present a digital output signal representative of an analog input signal.
By way of example and not by way of limitation, a sigma delta signal treating apparatus shall be described herein employing a representative circuitry referred to as a sigma delta converter. Sigma delta converters employ two principles in effecting analog-to-digital conversion of signals to present a digital output signal representative of an analog input signal: oversampling and noise shaping. Oversampling spreads quantization noise power over a bandwidth substantially equal to the sampling frequency, which is much greater than the signal bandwidth of the input analog signal. A modulator is employed in a sigma delta converter apparatus to effect low pass filtering of the analog input signal and to operate as a high pass filter of noise, thus “shaping” the quantization noise so that most of the noise energy will be higher than the signal bandwidth, also referred to as the band of interest. A subsequent digital low pass filtering stage substantially reduces out-of-band quantization noise, and final downsampling with a decimation filter brings the sampled signal to the Nyquist rate.
In high speed, or wideband sigma delta converter apparatuses using a higher oversampling ratio (OSR), the noise floor (i.e., a noise level which is substantially irreducible) is dominated by thermal noise that the input sampling circuit may generate while operating at a lower OSR. As the OSR rises or as the band of interest includes higher frequencies, the high frequency portion of the high-pass-shaped quantization noise begins to rise above the thermal noise floor and affects the overall signal-to-noise ratio (SNR) of the apparatus.
One approach to suppressing quantization noise in a sigma delta filter is described by R. Brewer, et al. in “A 100 dB SNR 2.5 MS/s Output Data Rate ΔΣ ADC”; 2005 IEEE International Solid State Circuits Conference, Session 9, Switched-Capacitor ΔΣ Modulators; Digest of Technical Papers; 2005 (hereinafter referred to as “Brewer”). Brewer discloses a ΔΣ modulator circuit having three stages and associated output circuitry. Brewer's first stage is a low pass signal transfer function stage having feed-forward topology. Brewer's second stage is a high pass noise transfer function stage having a feed-forward topology and performing noise shaping. Brewer's third stage is simply an analog-to-digital conversion unit that performs a noise shifting or lowering function without performing any noise shaping. Using a feed-forward topology involves some shortcomings, such as requiring precision of timing operations, requiring parts (e.g., amplifier units) having fast settling times to accommodate timing requirements, and other shortcomings.
The inventors recognized a need for a simpler and, therefore, less expensive, alternative to providing a sigma delta signal treatment apparatus that permits suppressing quantization noise. The inventors recognized that using a feedback topology in constructing a notch filter (referred to herein as a feedback notch filter) for use in a signal treatment apparatus, such as a sigma delta converter apparatus, is effective in relatively low noise environments. Using such a feedback notch filter requires less stringent timing criteria, less costly components and entails fewer design constraints. Using such a feedback notch filter in a relatively low noise environment is acceptable to a designer without requiring a separate stage in the apparatus for carrying out a noise lowering function.
There is a need for a simple, low cost apparatus and method for effecting sigma delta signal treatment that permits suppressing quantization noise to a level at or below white noise and thermal noise levels over an increased bandwidth than has previously been attained.